This invention relates to gaskets, and in particular to a gasket for a demountable pipe joint of an hygienic food processing system.
Systems and apparatus for the processing of liquid food inevitably require joints between adjacent pipes of the apparatus. Although pipes can be joined by welding, they can not then be readily disassembled for cleaning and other purposes.
In view of the foregoing, demountable pipe joints are preferred in many applications, although it is recognized that such joints (i.e. joints that are not welded together but can be assembled and disassembled) are unsatisfactory from a hygienic point of view. Problems associated with demountable pipe joints are discussed in a paper by the European Hygienic Equipment Design Group entitled xe2x80x9cHygienic Design of Closed Equipment for the Processing of Liquid Foodxe2x80x9d by G. J. Curiel, Dr. G. Hauser, P. Peschel and D. A. Timperley dated October, 1993. In this document, various types of pipe joint incorporating elastomeric gaskets are discussed. As a result, in general, it is clear that gaskets, whether in the form of sheets, moldings or xe2x80x9cOxe2x80x9d rings, which are usually made of elastomer such as Neoprene rubber, EDPM or Viton, are not ideal. These known prior art gaskets are used because they deform to take the shape of the mating surfaces when relatively small forces are applied. However, these gasket materials xe2x80x9ccreepxe2x80x9d, particularly at high temperatures, and compress or move readily during use in response to the pressure of liquid food in the pipe of the processing apparatus.
If a gasket exhibits xe2x80x9ccreepxe2x80x9d (xe2x80x94creep means a time dependent change of relaxed dimensions after having been subject to heat and/or pressure), the pressure between the gasket and a mating face of a pipe end at a joint can be lost and crevices can open up. As a result, micro organisms can become established in the crevices, thereby preventing the pipe joint remaining sterile. Furthermore, problems with micro organisms can be especially bad when using elastomeric gaskets because the food processing equipment usually goes through cycles of heating and cooling during long production runs and the micro organisms can easily flourish in crevices as the soft elastomeric gasket moves in its seat.
Another problem associated with elastomeric gaskets is that, when a pipe needs to be sterilized before a new production run is commenced, temperatures between 122xc2x0 and 140xc2x0 (for example) need to be applied, often by means of superheated pressurized water or steam to kill any micro-organisms present. If there are crevices present then the sterilizing agent may not contact the entire surface. Heating elastomers to this sort of temperature, however, results in degradation of the material, thereby shortening the life of the gasket.
U.S. Pat. No. 5,518,257 discloses a seal device which incorporates an inner seal member made from engineering plastics and resiliently biased towards a flow path. The inner seal member is designed to slide relative to pipe ends of a joint and includes a sharp internal corner, both of which are inherently unsatisfactory in hygienic applications due to their ability to harbor micro-organisms.
Although metal gaskets are known in certain applications, these are not considered appropriate in food processing apparatus because they can result in scars and scratches being produced on the pipe ends which, when a new gasket is applied, will prevent a satisfactory seal being produced.
Although the problems described above have been known for many years in the field of hygienic food processing equipment, as far as the applicant is aware no one has provided a satisfactory answer until now.
With the foregoing in mind, the present applicant has invented a new demountable pipe joint and gasket suitable therefor which overcome the problems associated with the prior art in a simple and efficient manner. Furthermore, although the present invention appears to go against all teachings of the prior art in this particular specialized field, it provides a significant improvement over the prior art.
According to the present invention, there is provided an apparatus for use in a hygienic food processing system, comprising a gasket made substantially of engineering plastics material, wherein said engineering plastics material is selected to be resistant to creep during said gasket""s use in said hygienic food processing system. According to a preferred embodiment the gasket comprises a bore surface for providing a transition from a first joint part to a second joint part at a demountable joint of said hygienic food processing system, and a side defining a seal surface, adjacent the bore surface, for abutting at least one of said first and second joint parts, wherein said seal surface deforms in use to define a plateau positioned on a nib which extends from a body of the gasket. In use the gasket produces an hygienic seal.
As will be appreciated, a gasket according to the present invention is a significant departure away from known prior art gaskets of the relevant type, which in general were elastomeric and soft. Furthermore, although particularly applicable to hygienic food processing systems, a gasket according to the present invention may be used in any demountable joint or other appropriate application. For example, the invention may find significant uses in the biotechnology or pharmaceutical industries where cleanliness is required.
Although those skilled in the art will understand the significance of the present invention, in particular the difference between elastomers and engineering plastics, the distinction can be clearly recognized from the differences in relative tensile strength/modulus. In particular, a typical elastomer has a tensile strength of 20-40 MPa and an elongation of 300-700% before breaking. This implies a tensile modulus of less than 10-20 MPa. In contrast, a typical engineering plastic has a tensile strength of 100-200 MPa and an elongation of 3-100%, thereby giving a tensile modulus in the range of 2400-10,000 MPa at room temperature. Further, a typical elastomer exhibits between 2-3 times as much linear thermal expansion (and hence between 8-27 times as much volume expansion) as an engineering plastics such as polyetheretherketone.
Preferably each seal surface is positioned on a nib or platform which extends from a body of the gasket. In a particular embodiment, wherein the gasket is substantially annular, each nib is also substantially annular.
In a preferred embodiment, each seal surface includes a deformable ridge for mating with imperfections in a joint part during production of the hygienic seal.
In an alternative embodiment, each pipe end may include a raised platform portion for gripping a simply shaped gasket, such that the raised platforms of the pipe ends replace the nibs of the gasket.
The nibs are preferably shaped and sized to exhibit creep during assembly of a joint, such that deformation of the nibs results in an extremely good seal being produced between the gasket and the adjacent joint part.
The body of the gasket is preferably shaped and sized to remain resilient (i.e. elastic) throughout the lifetime of the gasket. With this in mind, provided that the compression force applied to the gasket does not exceed the critical stress (in this specification, xe2x80x9ccritical stressxe2x80x9d means the maximum stress at which no relatively rapid permanent deformation occurs) of the particular plastics material forming the gasket, the body of the gasket can remain resilient such that the nibs of the gasket are continually biased into contact with the joint parts. This preferably applies at all times and at all working temperatures experienced by the gasket.
The radial width of each seal surface of the gasket may be less than 50%, preferably less than 22%, more preferably about 20%, of the radial width of the body of the gasket. Further, the axial length of each nib is preferably less than 5%, more preferably less than 2%, of the total axial length of the gasket. By using these preferred relative dimensions, normal usage of the gasket results in the nibs experiencing creep during assembly of a joint and the body of the gasket remaining resilient throughout the assembly and subsequent lifetime of the joint, thereby keeping the deformed portion in contact with the joint part and thus the seal tight despite thermal cycling. A markedly improved seal is thereby provided which should not require any subsequent tightening of the joint.
The gasket may include a stop for defining the maximum compression of the gasket during use. The stop may be made of metal, possibly stainless steel, ceramic or plastics material. If the stop if made of plastics, it may be formed integrally with the gasket and take the form of an annular outer portion of the gasket which defines oppositely facing planar surfaces.
The gasket may include a metal fence which defines the maximum radial extent of the gasket. If a fence is employed, the fence may be a stainless steel ring which abuts the gasket and acts as the stop mentioned above. As a result, the inner plastics part of the gasket cannot expand radially, but can expand axially if necessary. The amount of expansion will, however, be minimal in comparison to the prior art elastomeric gaskets.
Preferably the sides of the gasket adjacent the fence define raised surfaces which extend further axially than the fence. As a result, when the gasket is positioned for use and compressed in a pipe joint, for example, the fence acts as the stop and the plastics part of the gasket can only be compressed by a predetermined amount dictated by the axial height of the raised surfaces.
An annular trough may be formed on each side of a gasket between the plateau and the raised surface. Alternatively, in use, a cavity may be defined around this region of the gasket. In any event, in this region, the gasket is unsupported and allows expansion of the gasket, if necessary.
The plastics material of the gasket is preferably resistant to creep at temperatures encountered during use. In particular, the plastics material is preferably resistant to creep up to about 140xc2x0 C. It is envisaged that a gasket according to the present invention is likely to be used in a pipe joint which may be exposed to sterilization temperatures of between 122xc2x0 C. and 140xc2x0 C., and may undergo many thermal cycles.
Preferably the plastics material of the gasket is a hard plastic, such as polyetheretherketone (PEEK), polyethersulfone (PES), TORLON (Trade Mark), a polyamide-imide, or a polycarbonate. Other hard/tough plastics, such as polyphenylsulfone (e.g. RADEL from Amoco), Liquid Crystal Polymer (e.g. VECTRA from Hoechst, or ZENITE from Dupont) or polyimide (e.g. VESPEL from Dupont), known to those skilled in the relevant art may also be used, if appropriate.
Although it may ultimately not be the preferred form of gasket according to the invention, it is envisaged that the gasket may be made solely of PEEK or other hard/tough plastics material. Furthermore, such a gasket may be used not only in a pipe joint, but also in or as part of a housing or body of a valve, a pump or a heat exchanger, for example.
According to another aspect of the present invention, there is provided a pipe joint of an hygienic food processing system comprising a first pipe end, a second pipe end and a gasket as claimed herein compressed between the two pipe ends.
The pipe ends may be urged together by means of flanges soldered onto the outsides of the pipes. By soldering the flanges onto the pipes, welding at the pipe joint can be avoided altogether. As a result, the significant temperatures associated with welding are not needed, thereby preventing damage to the pipes themselves, which may be made from high grade austenitic stainless steel.
When using flanges soldered on the outsides of the pipes, contact between the soldering metals (possibly tin based) and food products passing through the pipes is best avoided. This can be readily achieved using a gasket according to the present invention, which does not move significantly during use yet provides a reliable seal against material from within the pipes reaching the flanges on the outsides of the pipes. Hence, the advantages of soldering the flanges onto the pipes can be realized.
Preferably alignment of the pipe ends is controlled by the flanges. As will be appreciated, if the thickness of a pipe wall is only between 1.2 and 1.6 millimeters, it can be extremely difficult to align a gasket accurately with the pipe ends. Furthermore, producing a good seal between two thin pipe walls and an elastomeric gasket is extremely difficult to achieve. The present invention clearly assists in overcoming such problems. With this in mind, an inside surface of one of flanges preferably assists in aligning the gasket.
In one embodiment, the flanges may be drawn together by means of a screw thread. Any other appropriate means for closing the pipe joint can, of course, be envisaged by those skilled in the relevant art.